1. Field of the Invention
The present invention relates to a contour restoration apparatus for restoring degraded contours of received images and improving the sharpness of reproduced images.
2. Description of the Related Art
In a conventional color television transmission system, the sharpness of images reproduced by the receivers is reduced because of the limited bandwidth of the transmission. For example, in the NTSC system the bandwidth of luminance signals is limited to the maximum frequency of 4.2 MHz. Therefore, even if a picture is taken by an ideal camera tube, the slope of a change in luminance levels of a built-up image from black to white or white to black at the receiver can not be increased beyond a certain value, and the contours of the image look slightly blurred.
In order to remove the blur of reproduced images, recent years have seen contour restoration apparatuses that generate the second-derivative of the received signal and add it to the received signal to achieve contour restoration.
In the following is described a prior contour restoration apparatus.
FIG. 34 shows a block diagram of the prior contour restoration apparatus. In FIG. 33, reference numeral 1 denotes an input terminal for video signals, 11 and 12 denote delay circuits that delay the signal for a predetermined period of time, 13, 14, and 15 denote multipliers, 16 and 18 denote adders, 17 denotes a gain controller that varies the contour restoration level, and 2 denotes an output terminal. FIG. 35 shows the waveforms of signals at points a to i in FIG. 34.
The operation of the contour restoration apparatus organized as above is described below with reference to FIGS. 33 and 34. First, a video signal input to the input terminal 1 is applied to the delay circuit 11 and the multiplier 13. The output signal of the delay circuit 11 is applied to the delay circuit 12, the multiplier 14, and the adder 18. The output signal of the delay circuit 12 is applied to the multiplier 15. For example, if a video signal at point a having a waveform (a) of FIG. 35 is input to the input terminal 1, then signals at points b and c output from the delay circuits 11 and 12 have respectively waveforms (b) and (c) of FIG. 34. The original input signal at point a and signals at points b and c output from the delay circuits 11 and 12 are applied to the multipliers 13, 14, and 15, respectively. If the coefficients of the multipliers 13, 14, and 15 are respectively Ka=-1, Kb=2, and Kc=-1, then signals at points d, e, and f output from the multipliers 13, 14, and 15 have respectively waveforms (d), (e), and (f) of FIG. 35. These three signals are added by the adder 16, which outputs the second derivative of the original input video signal as shown in FIG. 35 (g) at point g. The output signal of the adder 16 is applied to the gain controller 17, which adjusts the amplitude of the signal at any given gain. For example, if the gain is 2, the signal at point h output from the gain controller 17 has a waveform shown in FIG. 35 (h). The output signals of the gain controller 17 and the delay circuit 11 are added by the adder 18, which outputs a signal at point i having a waveform (i) of FIG. 35 through the output terminal 2. This output signal is the desired signal that has removed a blur of the contour.
Prior contour restoration apparatus thus improves the sharpness of contours by obtaining a signal having steep slopes in contour parts.
However, effects of contour restoration are small on gently-sloping contours, and an increased contour gradient is accompanied with an addition of undershoots and overshoots so that image sharpness is improved at the expense of unnatural black and white hems added to a contour.